Precision instruments such as a scanning tunneling microscope (STM) generally operate with ultra high vacuum. Positioning and repositioning of components is carried out remotely and requires special precautions for the in-vacuum procedures. Such repositioning may require high precision of placement. The STM, for example, can analyze surfaces down to atomic accuracy and has some very small components including a metal probing tip mounted only 0.1 mm above the sample surface. Descriptions of some of the mechanical requirements for a STM are given in an article "In Situ Tip Exchange Mechanism For The Demuth-type Scanning Tunneling Microscope" by K. Yokoyama, T. Hashizume, H. Tanaka, I. Sumita, M. Takao and T. Sakurai, J. Vac Sci. Technol. B9 (2), 623-625 (1991).
The tips are replaced frequently, preferably Without interrupting the vacuum. Therefore, advantageously a carrier is located in the chamber to carry a number of tips. In replacing a tip, the carrier is moved to a position adjacent to the tip holder. The carrier would have positions for the tips arranged so that selection of a tip is effected by a selected orientation of the carrier.
Magnetic retention of an assembly with the electrode tip is particularly useful in such an arrangement, because the carrier can merely position a tip assembly near a magnetic holder and retention is effected magnetically into the holder without gripping mechanisms being required. Similarly the carrier can have a simple slot or the like for connecting to the assembly to pull it away for disengagement from the holder. However, magnetic fields from any magnet in the holder would interfere with other surface analysis techniques generally associated with an STM, such as Auger electron spectroscopy and X-ray photoelectron spectroscopy. Simple, conventional magnetic shielding has been found to be insufficient, and additional shielding components would interfere in the utilization of a tip at a sample surface.
Therefore, an object of the invention is to provide a novel apparatus for retaining an exchangeable electrode assembly in position by a magnetically shielded magnet. A further object is to provide such an apparatus with highly efficient shielding of magnetic fields. Another object is to provide such an apparatus with efficient shielding without extraneous components at the electrode.